Grievous just before RotS was able to fight Mace on equal terms while almost completely immobilized. He's a highly skilled tactician that can calculate responses to advanced lightsaber styles after exchanging few blows. He fights fast enough to dodge blaster bolts in close quarters, and with all four arms he can exceed 20 strikes per second. He's strong enough to smash durasteel and send Jedi flying several meters with punches and kicks. He is durable enough to tank fire from starships with little apparent effect. And most importantly, he fights dirty.
Grievous wins.